Research Document - 2005/005

Diving characteristics and sightability estimates of eastern Arctic bowhead whales, Balaena mysticetus, based on satellite-linked telemetry

By Dueck, L.P., M.P. Hiede-Jørgensen, M.V. Jensen, and L.D. Postma

Abstract

Satellite-linked dive-recording instruments were deployed on four eastern Arctic bowhead whales. Two of the tagged animals were sexually mature females accompanied by calves. The other two animals were inferred to be an adult male and a juvenile female. Dive measurement data was received between July 5 and August 11, 2003. Tags reported data for 17-34 days for a total of 96 tag-days. Approximately 17,500 dives ≥8 m in depth were recorded. Dive rate ranged from 2.8 dives/hr to 30.7 dives/hr, and both dive rate and variance increased with day-of-year. All four whales dove to depths ≥100 m. The maximum recorded dive depth was 400 m. Most dives (59%) were to depths ≤12 m; only 4.2% were to depths >50 m. Mean dive duration ranged from 2.6 min to 8.1 min (mean = 5.0 min., S.E. = 1.1 min, n = 4). Whales spent most of their dive time (63-78% of time-at-depth) at dive depths ≤12 m (mean = 71%, S.E. = 3%, n = 4). Overall surface time (≤ 4 m depth) for individuals ranged from 19% to 35% (mean = 28%, S.E. = 4%, n = 4). Differences between whales were evident for surface time and certain dive characteristics; females accompanied by calves had the lowest mean dive duration and spent more time at the surface than the other whales. No differences in surface time or dive characteristics were observed for time-of-day. Overall sightability estimates, based on pooled surface time (above 4 m depth) and partitioned by week, were 40% prior to breakup of landfast ice and ranged from 21% to 29% for subsequent weeks in which at least three tags were active. Adjustments to sightability estimates for application to aerial surveys of bowhead whales in the eastern Arctic are discussed.

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